The quantity of rain taken at an annual means, is the greatest at the equator, and it lessens gradually to the poles; but there are fewer days of rain there, the number of which increase in proportion to the distance from it. The Journal de Physique contains the following observa tions : " From north latitude 12° to 43°, the mean number of rainy days is 78 ; from 43° to 46°, the mean number is 103; from 46° to 50, 134; and from 51° to 60°, 161." Winter often produces a greater number of rainy days than sum mer, though the quantity of rain is more considerable in the latter than in the for mer season : at Petersburgh rain and snow falls on an average 84 days of the winter, and the quantity amounts to about five inches ; on the contrary, the summer produces eleven inches in about the same number of days. Mountainous districts are subject to great falls of rain ; among the Andes, particularly, it rains almost inces santly, while the flat country of Egypt is consumed by endless drought. The rain gauge affords reason to suppose, that a greaterquantity of rain falls in the lower strata of the atmosphere than in those above, which may be accounted for bythe drops attracting vapour in their near ap proach to the earth, though it must be admitted, that Mr. Copland, of Dumfries, discovered the rain collected in the lower gauge was greatest when it continued falling for some time, and that the great est quantity was collected in the higher during short rains, or at the conclusion of lengthened ones.
As rain is known to fall at all hours of the day and night, and at every season of the year, it is apparent that it is caused by operations which prevail eternally, and without defined interruption. M. Toaldo seems to think that a greater quantity de scends in the night than the day ; and it is certain that a south wind produces more rain than any other, though it falls during the prevalence of every wind : heavy falls also occur in the most com plete calms. M. Cotte published a pa per in the Journal de Physique, from which it appears that the mean quantity of rain descending at 147 places, between latitude 11° and 60° north, is 34.7 inches. " Let us suppose then," observes Dr. Thompson, " (which cannot be very far from the truth) that the mean annual quantity of rain for the whole globe is 34 inches. The superficies of the globe consists of 170,981,012 square miles, or 686,401,498,471,475,200 square inches: the quantity of rain, therefore, falling annually will amount to 23,337,650,812,030,156,800 cubic inches, or somewhat more than 91,751 cubic miles of water." There are 52,745,253 square miles of dry land on the globe : consequently the annual amount of the quantity of rain de scending upon it will be 30,960 cubic miles. The sea is supposed to receive 13,140 cubic miles of water, which flows into it annually ; therefore it must sup ply an equal quantity by evaporation, or the land would be completely drained of every particle of moisture. Mr. Dalton estimates the quantity of rain falling in England at 31 inches.
Exclusive of the general appearance of vapour when condensed into clouds, there are other forms in which the exist ence of moisture in the atmosphere is ob servable, particularly the halo, a luminous Circle appearing under certain circumstan ces round the sun, moon, and stars. This has been almost universally ascribed to the rays of light issuing from those bodies passing through a frozen medium of hail or snow ; and that this may be the case admits of very little doubt; but it is equal ly probable, that the rays of the sun, breaking through an uniformly dense cloud, nearly exhausted by rain falling from it, may produce a similar effect on moisture in a fluid state, and this is de monstrated frequently by the sun appear ing through such clouds. The parhelia,
' or mock sun, is another phenomena, ef fected by the rays of the sun darted upon frozen or fluid particles of water on either side of that body ; but the exact manner in which this appearance origi nates cannot, for obvious reasons, be as certained.
A constant attendant upon each of the phenomena that we have attempted to illustrate is wind, the doctrine of which deserves every possible attention, as much of our comfort, and health, and commerce wholly depends upon it. Were it not for this agitation of the air, putrid effluvia, arising from the habitations of man, and from vegetable substances, besides the exhalations from water, would soon render it unfit for respiration, and a general mor tality would be the consequence. In this instance also the philosopher finds his progress arrested, and his research bounded by insurmountable obstacles ; still, however, there are many facts esta blished that are highly satisfactory. The temperate zones are not under the influ ence of as regular winds as between the tropics ; the trade wind prevails annually and regularly in those parts of the Pacific and Atlantic oceans which lie near the equator; it blows from the north-east within a few points on the north side of the equator, and from the south-east on the opposite side ; and the interval space of these separate winds is from the se cond to the fifth degree of north latitude, and within the limits just mentioned, where the wind may be said never to blow from the north or the south : but there are dreadful storms, and perfect calms, equally dangerous and perplexing to the mariner, who finds the force of the trade winds decline as be approaches their bomndary. Between the tenth and thirteenth degrees of south latitude the trade wind prevails in the Indian ocean ; but north of it there is a change every half year, when they blow in an opposite direction to their previous course : these are termed monsoons, and their change is constantly productive of variable airs and storms of extreme violence, which frequently continue from five to six weeks, during which period the naviga tion is very dangerous. The monsoons take place one on the south and the other on the north side of the equator in the Indian Ocean, and. they extend to the eastern coast of China, and the longitude of New-Holland, from Africa : they, how ever, suffer partial changes through lo cal circumstances. They are, besides, not altogether confined to the space just mentioned, as the wind blows from the east or north-east between September and April, and for the remainder of the year from the south-west, on the coast of Brazil, between Cape St. Augustine and the isle of St. Catherine. Having thus di rected the attention of the reader to this part of the subject, we shall pass to the prevailing winds of our native country, which were ascertained by order of the Royal Society of London, which learn. ed body published the following result in their Transactions—At London.